Herestauss' Treatise on the Unknown Inhabitants of the Ancient World (Magic Item)
Herestauss' Treatise is a magical speaking book, with blank pages. The user may ask the book a question on supernatural creatures (as defined in the GM's own campaign). The book will answer by opening and closing its cover like a mouth. The Treatise's knowledge is not complete. It will know an answer to a question based on the rarity of creature about which the user inquires. Typically this involves subtracting the creature's HD from 10 and multiplying by 10 to get a percent chance to know. For example, the book can answer questions on 1 HD zombies with 90% accuracy; on 4 HD gargoyles with 60% accuracy, etc. When the book knows the answer it will reply truthfully. When it does not know, it will provide a partially correct answer, or an incorrect answer, or no answer at all, at the GM's discretion.
Inspired by a book title appearing in Maupassant's short story "The Horla."